Protective coating for metalhydride based devices

ABSTRACT

A switchable mirror device includes active layer ( 4 ) wherein the active layer changes its optical properties by adding/removal of hydrogen and includes a Mg-transition metal layer, or an Y or rare earth based layer the active layer being provided on one side with a further protective layer ( 3 ) including a hydrogen and oxygen permeable and water impermeable layer, characterized in that the layer is liquid water impermeable and water vapor permeable and has hydrophobic surface properties. A pd catalyst layer ( 5 ) may be disposed between the active layer ( 4 ) and the protective layer ( 3 ).

The present invention relates to a thin film device according to the preamble of claim 1.

Such a thin film device is known from EP 1345071. In this patent specification a switchable mirror is disclosed wherein the active layer comprises Mg—Ni. The Mg—Ni metal layer is provided on a substrate and on the other side a catalyst or protective layer such as Pd can be provided. Optionally, the two layers are separated by a third layer, through which hydrogen can diffuse. The optical properties of the active layer depend on the hydrogen content and can be changed, e.g. from transparent to absorbing black to reflective by adding/removing hydrogen. These changes in properties are promising for many applications wherein a repeated change from transparent to reflective is desirable, such as smart windows or hydrogen sensors.

However it has been observed that reproducibility is a problem and at cycling either the quality of the desirable condition deteriorates or the process of switching slows down.

The subject invention aims to remove this drawback and to provide a thin film device in which an increased number of cycles can be effected without deterioration of the desired properties.

According to the invention this is realized with the features of claim 1.

According to the invention a protective layer is provided on the one hand allowing for free conveyance of hydrogen in and from the active layer but on the other hand blocking transport of liquids such as water. Furthermore, the surfaces of the further layer prevent the formation and attachment of liquid water droplets.

It is assumed that the improved properties obtained by the invention are based on the fact that for removal of hydrogen generally oxygen or an oxygen containing gas is used. Oxygen combines with hydrogen and the resulting water can hamper functioning of the active layer if it is in liquid condition. This water can result in corrosion because of the formation of e.g. hydroxides on the surface of the active layer. Water vapor should be able to migrate through the liquid water impermeable layer. Preferably the opening size in the additional layer according to the invention is such that water vapor and the standard conditions can migrate with a speed of 2.10⁻⁶ 1 H₂O vapor/cm².sec.

It is emphasized that the claimed invention is independent from the correctness of this theory.

Surprisingly it has been found that through the use of such further layer as protective layer a large number of cycles under the most severe circumstances of the thin film can be effected without deteriorating properties thereof. Also handling of the thin film is improved because the further layer provides protection of the active layer being there below. Also the effectiveness of the catalyst which is covered with the layer or coating according to the invention is not affected.

Effectiveness of the invention can be further improved by including scavengers in the protective layer. As example the presence of sulphur spheres is mentioned in the protective layer for scavenging sulphur in H₂S which poisons the catalyst below the protective layer.

Furthermore is has been observed that a further protective layer having a thickness between 10-100 nm does not have any negative effect on the rate of entry of hydrogen in the transition metal layer or its removal therefrom. Depending on the material of the further layer the thin film device can be used at both low and high temperatures.

According to a preferred embodiment of the invention the device is used as a optical switchable mirror. The transmission, reflection or absorbtion of the device is changed by adding or removal of hydrogen. In case of a change in transmission, the further layer transmits light in order to obtain the optical switching effect. In particular, the application could be in gasochromic smart windows based on GdMg-hydride as the active layer.

Alternatively, combinations are possible with solar collectors to control the light absorption and maintain the operation temperature within a required range. Such embodiments preferably comprise a Mg-transition metal alloy such as magnesium-Ti being arranged on a substrate, on which a catalyst layer comprising Pd is arranged. This catalyst can comprise pure Pd or mixtures thereof. For example Ag can be added in a quantity of for example 20-30 a %.

Depending on the hydrogen concentration the active layer switches from reflective to absorbing. The protective layer as discussed above is arranged on the Pd-material. The protective or further layer preferably comprises a plastic material and has a thickness of 10 nm to 1 μm and more particular about 50 nm.

It has been observed that by using the protective coating the stability of the films in basics solutions against delamination is extended to neutral and acid solutions.

The hydrophobic further layer according to the invention can be arranged on the thin film with any method known in the art. Preferably sputtering is used. More particular sputtering is used if Teflon is used as protective or further layer. Other methods of depositing are spin coating and so on. As indicated above Teflon (polytetra fluorethene) is a preferred material for use as protective or further layer.

Except from protection against the ingress of liquid water in the active layer through the use of a further or protective layer ingress of impurity molecules such as for example CH4 or large molecules such as those originating from plastics are prevented.

In the figure an example of the invention is shown.

The thin film device according to the invention is generally indicated by 1 and is provided on a substrate 2. This device comprises a magnesium transition layer 4 on which a catalyst Pd layer 5 is provided.

The hydrophobic layer according to the invention is indicated by 3 and is provided on top of both layers.

Although the invention has been disclosed referring to specific applications and specific lay out of the thin film device, persons skilled in the art will immediately realize that further developments are possible being within the scope of protection of subject application of which the scope of protection is defined by the following claims. 

1-14. (canceled)
 15. Switchable mirror device comprising active layer wherein said active layer changes its optical properties by adding/removal of hydrogen and comprising a Mg-transition metal layer, or an Y or rare earth based layer of which the properties change by adding/removal of hydrogen, said active layer being provided on a substrate and being provided on the other side with a further layer, further layer comprising a hydrogen and oxygen permeable and water impermeable layer, said layer being liquid water impermeable and water vapor permeable and having hydrophobic surface properties and wherein said further layer comprises a scavenger.
 16. Device according to claim 15, wherein said active layer comprises a Mg—Ti layer.
 17. Device according to claim 15, comprising a hydrogen sensor.
 18. Device according to claim 15, wherein said further layer is substantially light transmitting.
 19. Device according to claim 15, wherein said transition metal comprises Ni.
 20. Device according to claim 15, wherein said active layer comprises a Pd catalyst layer.
 21. Device according to claim 15 wherein a third layer, through which hydrogen can diffuse, is provided between the catalyst layer and the metal hydride.
 22. Device according to claim 15, wherein said protective layer has a thickness of 10 nm-1 μm.
 23. Device according to claim 15, wherein said protective layer comprises a polymer material.
 24. Device according to claim 23, wherein said polymer material comprises Teflon material.
 25. Device according to claim 15, wherein said scavenger is a sulphur scavenger.
 26. Device according to claim 15 wherein said scavenger comprises Ag.
 27. Method for producing a thin film device having an active layer comprising a Mg-transition metal layer or an Y or rare earth based layer which changes its properties by adding/removal of hydrogen, comprising the provision of a substrate, deposition of said active layer on said substrate and arrangement of a protective further layer on the outermost catalytic Pd layer, wherein said further layer is arranged by sputtering. 